Process for the production of high melting metals

ABSTRACT

PURE TUNGSTEN, NIOBIUM, TANTALUM, ZIRCONIUM, HAFNIUM, URANIUM AND OTHER HIGHER MELTING METALS ARE PREPARED BY REDUCTION OF AN ORE THEREOF TO PRODUCE AN ALLOY HAVING THE METAL IN SOLID SOLUTION. THIS ALLOY HAS A MELTING POINT BEING SUBSTANTIALLY LOWER THAN THAT OF THE PURE METAL. THIS ALLOY IS CRUSHED AND THE CONTAINED METALS ARE TRANSFORMED TO HALIDES WITH A HALOGEN. THE HALIDES ARE SEPARATED THEREAFTER AND THE MAJOR PART REDUCED TO METAL. ALTERNATIVELY TUNGSTEN IS PRODUCED BY TREATING THE ALLOY OBTAINED WITH A SUITABLE AGENT DISSOLVING THE ALLOYING AGENTS. THIS TUNGSTEN IS PARTICULARLY SUITABLE FOR PRODUCING TUNGSTEN CARBONYL.

United States Patent Office 3,701,649 Patented Oct. 31, 1972 3,701,649 PROCESS FOR THE PRODUCTION OF HIGH MELTING METALS Elis Kjell Ake Svanstrom and Lars Henry Ramqvist, Nynasllamn, Sweden, assignors to Rederiakliebolaget Nordstjernan, Nynashamn, Sweden No Drawing. Filed Apr. 4, 1969, Ser. No. 813,746 Claims priority, application Sweden, Apr. 9, 1968, 4,765/68; Feb. 7, 1969, 1,661/69 Int. Cl. C22!) 57/00 US. Cl. 75-845 6 Claims ABSTRACT OF THE DISCLOSURE Pure tungsten, niobium, tantalum, zirconium, hafnium, uranium and other higher melting metals are prepared by reduction of an ore thereof to produce an alloy having the metal in solid solution. This alloy has a melting point being substantially lower than that of the pure metal. This alloy is crushed and the contained metals are transformed to halides with a halogen. The halides are separated thereafter and the major part reduced to metal. Alternatively tungsten is produced by treating the alloy obtained with a suitable agent dissolving the alloying agents. This tungsten is particularly suitable for producing tungsten carbonyl.

BRIEF SUMMARY OF THE INVENTION The present invention relates to a process for producing refractory (high melting) metals, such as tungsten, niobium, tantalum, zirconium, hafnium, and uranium from ores thereof.

PRIOR ART At present tungsten powder is substantially reduced from ore by means of the so-called APT-process (ammonium paratungstate) This is a wet-chemical process, meaning that the ore is treated in several steps, inter alia by means of hydrochloric acid and ammonia followed by the precipitation of ammonium paratungstate, which is then transformed to tungsten trioxide and thereafter reduced to metal. The greatest difliculties of this process are the metallic impurities usually occurring in the ores, especially molybdenum. In order to separate these impurities several precipitations of, for instance, molybdate are required, which generally require new baths for each precipitation. To reduce the number of treatments relatively pure ores are selected, and these ores are often concentrated before applying the process. Because a plurality of steps are required in the foregoing process, generally a low yield is obtained, for example seldom more than 90% The disposal of the great amounts of chemical solutions poses a pollution problem of great magnitude, and moreover, the process usually requires comprehensive and bulky equipment.

This production process may be considered the general state of the art, though a few other methods also exist. All the processes have in common the fact that they are complicated and uneconomical and result in very high prices for the metals produced.

DETAILED DESCRIPTION The object of the present invention is to provide a process, in which a refractory (high-melting) metal is obtained with a very high yield, usually more than 99 or 99.5%, from an ore thereof. Another object is to provide a process for the production of refractory (high-melting) metals, which does not require space or personnel. Yet another object of the present invention is to provide a process for the production of refractory (high-melting) metals from oxidic ores thereof, which process is compact and can be easily controlled. A process for production of tungsten from ores thereof, which tungsten is especially suitable for production of carbonyl, is another object of this invention.

It will be apparent that these objects can be achieved by reducing ores of a refractory (high-melting) metal with such amounts of at least one reducing agent that the result formed has a melting point substantially lower than the melting point of the pure metal, crushing the alloy thus obtained and reacting the particulate metal with a halogen to transfer the metal to a halide thereof, from which the metal is reduced. It may be desirable to add to the alloy some additional substance, e.g. boron, to achieve sufiicient reduction of the melting point.

According to the present invention refractory metals, preferably tungsten, niobium and tantalum, are thus reduced from oxidic ores, which metals are important as raw material for the metallurgical industry, especially the hard metal industry. The need for these refractory metals is at present rapidly increasing. However the invention also relates to production of other refractory metals, such as zirconium, hafnium and uranium.

The present process may be said to consist of three reactions or steps. While each such reaction step is known per se, the combination of these steps provide an industrially useful process having essential and unexpected advantages. According to the present invention, an essentially simplified process is provided, the yield is almost one hundred percent; the process is carried out more rapidly and the degree of purity of the ores is of little consequence for carrying out the process and for the purity of the product. In addition, the invention has economic advantages. Tungsten prepared according to the present process has moreover the advantage of being substantially free of carbon. The carbon content in the tungsten alloy has often been a problem in production of tungsten.

In the first process step the ore is treated with one or several reducing agents, for instance silicon, aluminium and the like. The man of the art selects from a constitutional diagram for the reducing agent and the metal a suitable temperature and a corresponding suitable composition of the alloy he wishes to obtain. In case of silicon the content of silicon in the alloy may for instance be at least 8 percent by weight, preferably 9-20 percent. The alloy obtained is then reacted with a halogen, preferably chlorine, to form metal halides. The metal halides obtained are preferably separated by distillation. Alternatively, in certain cases a primary separation may be carried out by means of fractionated condensation before the distillation step. The purified metal halides are then reduced to metal. Examples of reducing agents are hydrogen gas, alkali or earth alkali metals. The reduction may also be carried out by melt electrolysis.

In case of a tungsten alloy the alloy obtained may be alternatively treated with a suitable agent, for instance hydrogen chloride, to dissolve the alloy substances. Tungsten obtained in this way is especially suitable as starting material for production of tungsten carbonyl.

This invention will be explained more in detail in the following with reference to tungsten, but it is obvious that the other metals indicated may be prepared analogously.

The raw material comprises oxidic tungsten ore, such as scheelite, wolframite or hiibnerite, and these ores may possibly be contaminated by molybdenum, tin, arsenic etc. The ore is finely crushed to simplify the reduction.

The reduction is preferably carried out in an electric arc furnace consisting of a graphite crucible and graphite electrode. The furnace is heated to operating temperature, i.e. 1800-2200 C., by means of a slag, and the ore mixed with a predetermined amount of reducing agent is continuously added. The selected reducing agents do not give off any gases and this condition as well as the fact that the reductions are exothermic, permit a high production speed. Charging of the reaction components goes on until the furnace is filled with melt up to a suitable level. Sometimes it may however be suitable to interrupt charging in order to tap the slag. The great difference in bulk density of the metal phase and the slag will cause the metal phase to sink very rapidly to the bottom of the furnace and a cleanly defined separation between metal phase and slag is obtained. When the furnace has been filled to a suitable level the process is interrupted and the melt is permitted to solidify before it is taken out from the furnace. The casting is cooled and separated from the slag.

The reducing agent preferably consists of silicon in case of the tungsten ores. It has appeared that the reduction with silicon can be used for all tungsten ores, even if they are badly contaminated.

The amount of silicon is preferably predetermined so that the reduced melt as to its composition will be between the phases W Si and WSi and a composition being between the phase W Si which corresponds to 37.5 atomic percent of silicon, and the eutectic point, which corresponds to 59.5 atomic percent of silicon, is particularly suitable. It has quite surprisingly appeared that a tungsten silicide is a very suitable starting material in the next process step, chlorination. Tungsten silicide is very easily chlorinated, and the two-phase structure results, in addition to this, in an increase of the chlorination speed in comparison with a one-phase structure. The phase rich in silicon is first attacked and sets free a larger surface of the other phase for further attacks.

As reducing agent for the tungsten ores aluminium may also be used.

Generally each reducing agent or combinations of reducing agents and other substances may be used, which form an alloy with tungsten or other refractory metals, the melting point of which alloy relative to the melting point of the pure metal is considerably decreased. It is for instance known that niobium and tantalum form lowmelting alloys with small amounts of boron. Such alloys are preferably achieved by reducing the ore by means of aluminium in the presence of boron oxide. The obtained alloy containing boron is also a suitable starting material for the next process step, chlorination.

In a series of reduction tests made with varying contents of silicon and aluminium within the composition range indicated above it appeared that no considerable tungsten content was present in the slag. This first process step can thus be carried out with a yield of almost one hundred percent. The speed of the reduction and the relatively simple and compact equipment required for this process step should moreover be stressed, which contributes to a good economy.

Chlorination is the next process step. The castings of the tungsten silicide and alloys of other refractory metals are crushed to a piece size of some centimeters and are introduced in a reactor. The chlorination process passes off quickly at a temperature of 400-500 C., and this temperature must at the start of the reactor be brought about in some way or other. The chlorination process is however very exothermic, which means that the reaction after start must rather be cooled ofl. The reactor may then operate continuously by preferably adding the silicides and chlorine gas at the top of the reactor, and the chlorides obtained in gas phase are taken out at the bottom of the reactor. Also in this process step the design of the apparatus is generally very simple. Nor does any loss of tungsten occur. Small residues of slag that may have accomplished the silicides are the only remaining material in the reactor.

The tungsten chloride obtained may be of the type hexa-, pentaor oxy-chloride. At the same time chlorides of silicon, aluminium, molybdenum, iron, boron, etc. are obtained. It should also be mentioned that the chlorinating ability of the tungsten silicides are not very much influenced by the metallic contaminations.

It is known that tests have been made to chlorinate tungsten ore directly. Such tests, however, have not been successful, above all depending on the fact that the presence of veinstones and ore components that cannot be chlorinated have hampered the chlorination process and obstructed the possibility of achieving good yields. The large amounts of chlorination residues in the reactor are also a problem in practical handling. This stresses the idea of the invention, i.e. that tungsten should be first transformed to a silicide or another alloy, after which chlorination can be advantageously made.

Although chlorine is most inexpensive and the best agent to use in treating the tungsten silicide, also other halogens, such as fluorine and bromine are quite useful.

The obtained chloride gases are condensed and lead into the third process step, that is distillation.

In the first fraction, chlorides of silicon, aluminium, iron, molybdenum and boron are taken out at temperatures of up to the boiling point of the tungsten chloride. In the next fraction at somewhat higher temperature, very pure tungsten chlorides are obtained. Small amounts of tungsten chlorides accompanying the first fraction may be easily recycled in the process. This assures almost a one hundred percent yield of tungsten in the whole process chain.

The chlorides of Si, Al and B are very easy to separate from the tungsten chloride. They are valuable by-products per se. Also the obtained tungsten chloride may well be a valuable end product in this form.

However, the tungsten powder is rapidly obtained by leading the tungsten chlorides directly from the distillation into a reactor simultaneously with hydrogen gas. The hydrochloric gas thus formed may in certain cases by recycled in the process to the chlorination vessel. At chlorination with hydrochloric acid gas hydrogen gas is set free, which may again be used in the end phase.

Also other methods of reducing the metal powder from the metal chloride exist, for instance melt electrolysis, reduction with alkali metals etc. These methods are particularly suitable for the metals zirconium, hafnium and uranium. The niobium and tantalum chlorides may be easily reduced by means of any of said methods.

The tungsten alloy obtained in the present process may be alternatively crushed to form particulate material of such a low grain size that a dissolution of the alloy substance near to the center of the grain will be possible to carry out in practice. For economical reasons crushing should of course not be too extensive. On the other hand a larger grain size requires a longer dissolution time. Pure, reactive, porous tungsten is then obtained by means of selective dissolution of the alloy materials, i.e. usually silicon or aluminium, from the alloy by for instance hydrogen chloride at elevated temperature.

In this way raw tungsten is obtained that has a great specific surface. This surface is fresh and reactive and the tungsten obtained in this way is therefore particularly suitable as starting material for preparation of tungsten carbonyl. The carbonyl formation is a surface reaction controlled by the absorption of CO and the desorption of carbonyl formed. Like several surface reactions the carbonyl formation is sensitive to contaminations. Traces of oxygen thus prevent the formation of carbonyls, whereas traces of sulphur or iodide may strongly promote the reaction. Therefore it is essential that the starting material for the production of carbonyl is in all respects as suitable as possible. This is achieved by the indicated method.

Especially pure tungsten powder may be produced from obtained tungsten carbonyl. The production from the obtained tungsten powder can take place in a way known per se, for instance by reaction with CO in an autoclave at elevated temperature and under pressure.

The tungsten powder produced may also be used in other connections, for instance as starting material for production of tungsten carbide.

The invention will be described in the following examples, which are not intended to limit the invention but should only illustrate it.

Example 1 1 kg. finely crushed scheelite ore containing 70.5% of W was mixed with 210 g. finely crushed silicon of 98% and added continuously to the crucible in an electric arc furnace, which was kept at about 2000 C. Tungsten was reduced, which combined with the excess of silicon to form tungsten silicide. The tungsten sank to the bottom of the crucible and formed a solid slab or ingot (casting) cleanly separated from the slag obtained at the same time. Analysis of the tungsten silicide showed that it contained 45 mol percent of silicon and about 55 mol percent of tungsten. The liquidus point of a silicide with this composition lies at about 2350 C. Analysis of the slag showed that it contained 0.4% of tungsten.

After cooling, the slab was removed from the crucible, the slag was chiseled out and the slab was crushed to provide particulate material as big as hazel-nuts. This material was transferred to a chlorination furnace and chlorinated at 800 C. Tungsten chloride silicon tetrachloride formed were primarily separated by fractional condensation, after which the tungsten chloride was purified from small amounts of contaminations by fractionated distillation, and the pure chloride was reduced in gas phase by means of hydrogen gas.

Example 2 The same test was repeated but with the difference that silicon was replaced by aluminium powder. The reaction mixture consisted of 1 kg. scheelite ore and 152 g. aluminium. Also in this case a metal slab substantially free of pores was separated on the bottom of the crucible and the slag formed contained 0.4% of tungsten. The metal slab contained 60 mol percent of aluminium and about 40 mol percent of tungsten.

The slab was then treated in the same way as the tungsten silicide. Chlorination and fractional condensation gave an impure tungsten chloride, which after purification by fractional condensation was reduced to metal powder by means of hydrogen gas.

What We claim is:

1. In a process for the production of tungsten metal from oxidic tungsten ore, the improvement which comprises,

6 reducing said ore at an elevated temperature with a reducing agent comprising silicon in an amount at least sufiicient to form a tungsten-base alloy containing tungsten silicide and having a melting point below the melting point of tungsten, solidifying said tungsten alloy, crushing said tungsten alloy, chlorinating said crushed tungsten alloy at an elevated chlorinating temperature whereby to convert at least said tungsten to volatile tungsten chloride,

recovering said tungsten chloride by fractional distillation,

and thereafter reducing the tungsten chloride to tungsten metal.

2. The process of claim 1, wherein the amount of silicon is at least sufficient to form a tungsten alloy containing at least about 8% silicon by weight.

3. The process of claim 2, wherein the amount of silicon in the reducing agent is sufiicient to provide a tungsten alloy containing about 9% to 20% by weight of silicon.

4. The process of claim 1, wherein in addition to the reducing agent, an alloying material is added to further lower the melting point of the tungsten alloy.

5. The process of claim 4, wherein said additional material comprises boron.

6. The process of calim 1, wherein the tungsten chloride is reduced to tungsten powder with hydrogen.

References Cited UNITED STATES PATENTS 3,460,937 8/1969 Rathmann 84 3,406,056 10/1968 Albert et al. 75-84 3,295,921 1/1967 Callow et al. 23-87 OTHER REFERENCES Extraction and Refining of the Rarer Metals, Stephen Austin and Sons Ltd., 1957, pp. 267-8. (TN 798154).

CARL D. QUARFORTH, Primary Examiner B. H. HUNT, Assistant Examiner US. Cl. X.R. 

